Biogenic Assemblage and Hydrodynamic Settings of the Tidally Dominated Reef Platform Sediments of the Zanzibar Channel
Total Page：16
File Type：pdf, Size：1020Kb
Biogenic Assemblage and Hydrodynamic Settings of the Tidally Dominated Reef Platform Sediments of the Zanzibar Channel Item Type Journal Contribution Authors Shaghude, Y.W.; Wannas, K.O.; Mahongo, S.B. Citation Western Indian Ocean J. Marine Science, 1 (2), p. 107-116 Publisher Western Indian Ocean Marine Science Association (WIOMSA) Download date 02/10/2021 19:24:13 Link to Item http://hdl.handle.net/1834/32 Western Indian OceanHYDRODYNAMICS J. Mar. Sci. Vol. OF 1,TIDAL No. 2,REEF pp. SEDIMENTS 107–116, 2002 OF ZANZIBAR CHANNEL 107 © 2002 WIOMSA BiogenicAssemblageandHydrodynamicSettingsofthe TidallyDominatedReefPlatformSedimentsofthe ZanzibarChannel YohannaW.Shaghude1,KjellO.Wannäs2andShigallaB.Mahongo3 1Institute of Marine Sciences, University of Dar es Salaam, P.O. Box 668, Zanzibar, Tanzania; 2Department of Geology and Geochemistry, Stockholm University, S-106 91 Stockholm, Sweden; 3Tanzania Fisheries Research Institute, P.O. Box 9750, Dar es Salaam, Tanzania Key words: biogenic assemblage, hydrodynamics, carbonate sediments, reef platforms, Zanzibar channel, flood currents, ebb currents Abstract—The biogenic assemblage and hydrodynamic settings of the tidally dominated reef platform sediments (TDRPS) east of the Zanzibar channel were investigated in order to characterise the carbonate sediments. Benthic foraminifera were found to be the most important group, both in terms of average abundance (> 60 % by weight) and also in terms of spatial distribution, with common occurrence in all sediments samples. Pelecypods, with an average abundance of 8 % were slightly less widely distributed. The remaining groups occurred at lower average abundance and less frequently in the sediments. Current measurements indicated that the maximum current speed is higher during spring than during neap tides. The direction is phase-dependent with flood tidal current flowing from the northern and southern tips of the island towards Zanzibar town. The ebb current flows in the opposite direction. The geomorphological setting of the carbonate platforms together with the present investigation of the currents and biogenic assemblage of the clastic sediments permits us to subdivide the reef platform area in the Zanzibar Channel into two sub-provinces: the northern and the southern. The northern sub-province contains significantly higher proportion of lithogenic particles, pelecypods, gastropod and ostracods than the southern sub-province. Some samples from the southern sub- province had significantly higher proportion of corals, than those from northern sub-province. INTRODUCTION aragonitic algae (Halimeda spp.) and many others. Second, the non-autotrophic benthic organisms, Tides, waves, currents and occasional storms are which use manufactured organic carbon for their all of major importance to the compositional survival. These include both the sessile and abundance of various biogenic groups in shallow sedentary organisms. The sessile organisms include water carbonate depositional systems (Boothroyd, molluscs, foraminifera, echinoderms, sponges and 1985; Jones & Hunter, 1992; Scoffin, 1993; Li et ostracods. Reef building corals and bryozoans are al., 1997; 1998). Shallow water carbonate among the well known sedentary organisms. sediments are mainly produced by two major Different carbonate-producing organisms groups of living organisms. First, the autotrophic flourish well if specific oceanographic conditions primary producers, which can utilise light energy are satisfied. Corals, for instance, thrive in shallow from the sun to make organic carbon. These include water characterised by high agitative organisms such as the coralline algae, the environments, where there is minimum siltation Corresponding author: YWS. E-mail: [email protected] 108 Y.W. SHAGHUDE ET AL. of fine sediments (Muhando, 1997; Wagner, 1999). siliciclastic sediments found on the beaches of the On the contrary, Halimeda spp., composed of west coast of the island (Shaghude & Wannäs, perforated segments which disintegrate into fine 1998; 2000). The main part of the island consists particles after death (Harney et al., 2000), are often of coralline limestone formations of Pleistocene associated with lagoonal environments where age (Kent et al., 1971). energy is low. Thus, since most shallow-water The tides around the island are semi-diurnal carbonate sediments are of intrabasinal origin with spring tidal range of 3.2 m and neap tidal (Swinchart, 1965), the oceanographic conditions range of 0.9 m (Cederlöf et al., 1995). Thus, the directly influence the biogenic assemblage of the area can be classified as mesotidal coast bioclasts. (Boothroyd, 1985). The most important current in The compositional assemblage of sediments in the Zanzibar channel is the East African Coastal shallow water carbonate depositional basins is Current (EACC), which has a net northward flow. rarely composed of 100 % bioclasts. This is The speed of this current varies between 0.25 and because most of these basins occur near continental 2 m/s, being fastest during the SE monsoon, and land masses where terrigenous materials may be lowest during the NE monsoon (Newell, 1957). introduced into the basins. On land, the terrigenous A bathymetric study conducted in one third of material (lithoclasts) is transported mainly by the channel area (Fig. 1a) shows that the channel rivers, wind or glaciers. When it enters the coastal is generally shallow and consists of a ‘central deep’ ocean, the material is subject to the influence of of between 30 and 40 m depth (Shaghude & currents, waves and winds, which mix them with Wannäs, 1998). The topography east of the central the bioclastic sediments. Grain size and proportion deep is irregular as is dominated by Pleistocene of the lithoclasts deposited in a particular location raised coral platforms, with a network of ‘internal’ depend upon various factors, including the distance channels. In the charts (e.g. Crown, 1995) the from the source, the energy of the depositing internal channels are labelled as Passes (Fig. 1b). medium and the general current pattern. Thus, Sediment characteristics of the Zanzibar channel oceanographic processes influence the ratio of and the biogenic, and mineralogical composition bioclastic: lithoclastic components in the sediments. of the sediments has been discussed by Shaghude The present study focuses on characterisation & Wannäs (1998; 2000), where they proposed a of the carbonate sediments of the tidally dominated division of the sediments into three bio- reef platform sediments (TDRPS) east of the physiographic provinces: 1- the coastal zone, 2- Zanzibar channel. the central channel zone and 3- the reef platform/ patch reef zone. The higher sampling density on MATERIALSANDMETHODS the eastern side allows further characterisation of the sediments in relation to the energy regime in Studyarea this area. In this study current measurements taken from Zanzibar (Unguja) Island (Fig. 1a) is one of the the reef platform system were used to relate the major islands of Tanzania located along the eastern biogenic assemblages of the carbonate sediments coast of Africa in the western Indian Ocean. The off the western coast of Zanzibar to the energy island is approximately north-south oriented, about regime. 85 km long and 35 km wide, with an area of 1530 km2. It is part of the inner shelf which was uplifted Sampleanddatacollection during Early to Mid Eocene (Kent et al., 1971). Unguja Island is fairly flat with only minor The sediment data consisted of 27 samples (Fig. relief on the northwestern side, where the 1b). All the samples were wet-sieved into four class Masingini Ridge rises about 100 metres above categories: very coarse sand (1.0 to 2.0 mm), coarse mean sea level (Kent et al., 1971). The ridge is sand (1.0 to 0.5 mm), medium sand (0.5 to 0.25 composed of uplifted sandstone sequences, which mm) and the fine grade material (< 0.25 mm). The are unique for the island and could be a source of last class (< 0.25 mm) was not counted, as these (b) HYDRODYNAMICS OFTIDALREEF SEDIMENTSHYDRODYNAMICS OFZANZIBARCHANNEL 109 Fig. 1. The location map (Fig. 1a) showing the Unguja island and the approximate boundary of the study area. The location of the samples investigated (indicated by + sign) and the Self Recording Current meter (SRC) stations (indicated by symbol) are shown in Fig. 1b. The labelled passes (Fig. 1b) are: EP (English Pass), FP (French Pass), GP (Great Pass), WP (Western Pass), SP (Southern Pass) and IP (Inner Pass) 110 Y.W. SHAGHUDE ET AL. grains were difficult to identify under a binocular Table 1. Results of a significant test for comparing microscope. Using a binocular microscope, the concentration of the different groups in the three sand sized classes (P = associated P-value) aliquots of approximately 300 grains of the other three classes were used to quantify different Group P Remarks biogenic groups. Tidal currents were taken from 6 stations (Fig. 1b) using a self-recording current Foraminifera 0.89 (*) No significant changes meter (RCM), Sensordata SD6000. In the present Pelecypods < 0.0001 Average decreases with study, the six stations were located in the passes decreasing size between the reef platforms. At each station, the Gastropods 0.1 Average decreases with RCM was deployed at a single depth, 5 m above decreasing size the bottom for about 5–6 hours, during peak spring Corals 0.96 (*) No significant changes and peak neap tides. Ostracods < 0.0001 Dominant in medium sand RESULTS Lithogens 0.1 Average increases with decreasing